The present invention is directed generally to injection molding methods and machines and more particularly to a device and method for reducing the leakage of liquid past the piston in an injection molding machine.
One conventional method used to produce molded metal alloys is the die cast method. In this method, the metal to be injected is heated above its liquidus temperature and then forced into the mold by the extension of a piston in the injection chamber. Another conventional method is the thixotropic injection molding method. In this method, the metal is first heated to a thixotropic state rather than to a completely liquid state, and then injected into a mold from an injection chamber. In this method, a screw rather than a piston is often used to inject the metal into the mold. The piston and the screw contain a shaft portion, which is attached to a drive mechanism. The drive mechanism is typically a motor, however, hydraulic mechanisms have also been used.
Although conventional liquid metal injection molding and thixotropic metal injection molding have been used successfully in the past, conventional machines suffer from metal leaking backwards past the piston or screw into the drive mechanism, rather than being injected forward into the mold. This occurs because high pressure is required to force the metal completely into the mold and it impossible to manufacture an apparatus without some clearance between the piston or screw and the inner wall of the injection chamber. Thus, the metal is forced past the piston or screw into the shaft housing. Some of this metal may reach the driving mechanism of the piston or screw and damage it. Thus, a device reducing the leakage of metal to the rear of the shaft housing in the injection molding of liquid and thixotropic metals is desired.
A preferred embodiment present invention provides an injection molding apparatus, comprising an injection chamber, an injection member comprising a head and a shaft located in the injection chamber, and at least one protrusion adjacent to the shaft.
Another preferred embodiment of the present invention provides an injection molding apparatus, comprising an injection chamber comprising a shaft housing portion having a polygonal internal cross section, and an injection member comprising a head and a shaft located in the injection chamber.
Another preferred embodiment of the present invention provides an injection molding apparatus, comprising an injection chamber comprising an accumulation portion and a shaft housing portion, at least one opening in a side wall of the shaft housing portion of the injection chamber, and an injection member comprising a head and a shaft located in the injection chamber.
Another preferred embodiment of the present invention provides an injection molding apparatus, comprising an injection chamber comprising an accumulation portion and a shaft housing portion, an injection member comprising a head and a shaft located in the injection chamber, a first means for heating the accumulation portion of the injection chamber to a first temperature above a liquidus temperature of a metal injected by the injection member, and a second means for heating the shaft housing portion of the injection chamber to a second temperature below a liquidus temperature of the metal injected by the injection member.
Another preferred embodiment of the present invention provides an injection molding apparatus, comprising a piston comprising a head and a shaft, the shaft having a diameter smaller than a diameter of the head, a first injection chamber portion having a front end, a back end and a cavity having a circular internal cross section and containing the piston head. The apparatus further comprises a second injection chamber portion joined to the back end of the first chamber portion, the second chamber portion having a front end, a back end and a cavity having a polygonal internal cross section and containing the piston shaft, at least one opening in at least one side wall of the second injection chamber portion, and at least one piston ring circumscribing the piston shaft.
Another preferred embodiment of the present invention provides a method of making an injection molded metal part, comprising providing a metal into an accumulation portion of an injection chamber, maintaining the accumulation portion of the injection chamber at a first temperature above the liquidus temperature of the metal, maintaining a shaft portion of the injection chamber at a second temperature below the liquidus temperature of the metal, and advancing an injection member having a head and a shaft in the injection chamber to inject the metal from the accumulation portion into a mold cavity.